While ripping out an old fireplace at a derelict house in Surrey, David and Ann Martin found what they thought to be a discarded chicken bone, but which on closer inspection turned out to be the remains of a pigeon.

Attached to the bird’s foot was a small red cylinder, and inside they found a coded message that was almost certainly dispatched from Nazi-occupied France. The highly sensitive Pigeon Service document was analysed by codebreakers at the Government listening post GCHQ, but they were unable to decode it, as it was almost certainly a one-time use code.

The contents of the message, which is not dated, may forever remain a mystery.